Use enum for hook names

[mostafa]

This PR removes string constants used for hook names with protobuf enums. It is now hard to get them wrong.
There are two compromises:

1. The enum names are generated from protobuf file, and are very long, which is okay-ish.
2. The plugin system currently doesn't support it, so when using enum values in PluginConfig["hooks"], they should be explicitly converted to int32. This will be fixed in Configuration management gatewayd-plugin-sdk#3.

Closes #190.